This thread https://www.qtcentre.org/threads/512...use-on-qwtpolt gave me insights. I don't really have time to post my solution. In short use a polygonal picker, after drawing the polygon, this last can send you the points via a signal. Then just store these points (with the proper transformation to axis values) in QPolygonF and feed this last to a QPainterPath. Create a QwtPlotShapeItem and use the QPainterPath to define its shape.